*Please note this is a story related acievement and therefore unmissable if you follow the Dawnguard story*
*Possible Spoilers Below*
To gain access to the Dawnguard questline you'll need to be level 10 and head to a hold and speak to some of the guards about recruiting vampire hunters (there are other ways, like asking the inn keeper about rumours, this is the easiest).
Once you've started the quest 'Dawnguard' , you'll recieve a quest marker southeast of riften. Head over there and enter the cave (for me, the quest marker on the map and the actual location I had to go to were different..)
Follow the path up to the Fort and inside you'll see the Fort Master arguing with a vigilante, approach them. You now have the option of joining Dawnguard! Accept it and you'll be given a quest to investigate Dimhollow Crypt.
Fast travel to the nearest location to the crypt (southwest of Dawnstar). Once inside, make your way through the cave fighting Draugrs and Vampires until you come to a button. (yes, just a button).. Heres where it gets interesting.. Click the button.. After the quick scene the place will light up.. Look around and you will see 6 or so stands with dust in them. Move them around until the dust in each one is on fire. The button will rise and you will have to active the stone obelisk. After an interesting scene you'll have what you came for! (and a pretty hot new friend ;) )
Beep Boop, Achievement Unlocked!
NOTE: A second save from before pressing the button might be a good idea as the quest might glitch and not grant you an achievement! (Credit to;Golgo28)
IF YOUR ACHIEVEMENT SHOWS AS UNLOCKED ON YOUR XBOX BUT DOESN'T SHOW UP ON TRUEACHIEVEMENTS
Re-download your profile (guide button>recover profile>enter account info(must be signed out)) . Once your profile is redownloaded you will have to RE-EARN the achievement, better hope you had an earlier save (; (Credit to; TIBERIUS Oni)
If you have any feedback/ways to improve this solution, give me a shout in the comments.